The invention relates to an improved method for forming hollow articles from injection molded thermoplastic preforms by subjecting selected portions of said preform to an annealing step prior to thermoforming into said hollow article.
It is known to thermoform molded elongated, tubular thermoplastic preforms into hollow articles such as bottles and containers. One particularly desirable method for producing the preforms that are used is by injection molding since such method allows for the high production of parts having very uniform dimensions and weight distribution. However, one problem which has resulted when using injection molded preforms is that upon heating said preforms to thermoforming temperature, significant lean (bending) and shrinkage occurs. This lean causes the thermoformed articles prepared from such preforms to have an undesirable variation in wall thickness and oftentimes the minimum acceptable limits will not be satisfied. This problem was not completely unexpected since injection molded parts are known to possess internal residual stresses due to the nature of the process, the equipment design and the geometry of the molded part itself (note "Injection Molding, Theory and Practice" by Irvin I. Rubin, Chapter 3, l972). However, despite the careful control and selection of process conditions and equipment design, the problem did not lend itself to an easy solution.
It is known that built in stresses can be relieved to some extent by annealing as disclosed in Rubin cited above. However, it was found that annealing of the entire elongated preform body did not give a satisfactory solution since significant lean still resulted upon heat up of the part.